AD781
D EFINITIO NS O F SP ECIFICATIO NS
Signal-To-Noise and D istor tion (S/N+D ) Ratio—S/N+D is
the ratio of the rms value of the measured input signal to the
rms sum of all other spectral components below the Nyquist
frequency, including harmonics but excluding dc. T he value for
S/N+D is expressed in decibels.
Acquisition Tim e—T he length of time that the SHA must
remain in the sample mode in order to acquire a full-scale input
step to a given level of accuracy.
Sm all Signal Bandwidth—T he frequency at which the held
output amplitude is 3 dB below the input amplitude, under an
input condition of a 100 mV p-p sine wave.
Total H ar m onic D istor tion (TH D )—T HD is the ratio of the
rms sum of the first six harmonic components to the rms value
of the measured input signal and is expressed as a percentage or
in decibels.
Full P ower Bandwidth—T he frequency at which the held
output amplitude is 3 dB below the input amplitude, under an
input condition of a 10 V p-p sine wave.
Inter m odulation D istor tion (IMD )—With inputs consisting
of sine waves at two frequencies, fa and fb, any device with
nonlinearities will create distortion products, of order (m+n), at
sum and difference frequency of mfa±nfb, where m, n = 0, 1, 2,
3.... Intermodulation terms are those for which m or n is not
equal to zero. For example, the second order terms are (fa+fb)
and (fa–fb), and the third order terms are (2fa+fb), (2fa–fb),
(fa+2fb) and (fa–2fb). T he IMD products are expressed as the
decibel ratio of the rms sum of the measured input signals to the
rms sum of the distortion terms. T he two signals are of equal
amplitude, and peak value of their sums is –0.5 dB from full
scale. T he IMD products are normalized to a 0 dB input signal.
Effective Aper tur e D elay—T he difference between the switch
delay and the analog delay of the SHA channel. A negative
number indicates that the analog portion of the overall delay is
greater than the switch portion. T his effective delay represents
the point in time, relative to the hold command, that the input
signal will be sampled.
Aper tur e Jitter—T he variations in aperture delay for
successive samples. Aperture jitter puts an upper limit on the
maximum frequency that can be accurately sampled.
H old Settling Tim e—T he time required for the output to
settle to within a specified level of accuracy of its final held value
after the hold command has been given.
FUNCTIO NAL D ESCRIP TIO N
T he AD781 is a complete sample-and hold amplifier that
provides high speed sampling to 12-bit accuracy in less than
700 ns.
D r oop Rate—T he drift in output voltage while in the hold
mode.
Feedthr ough—T he attenuated version of a changing input
signal that appears at the output when the SHA is in the hold
mode.
T he AD781 is completely self-contained, including an on-chip
hold capacitor, and requires no external components or
adjustments to perform the sampling function. Both input and
output are treated as a single-ended signal, referred to common.
H old Mode O ffset—T he difference between the input signal
and the held output. T his offset term applies only in the hold
mode and includes the error caused by charge injection and all
other internal offsets. It is specified for an input of 0 V.
T he AD781 utilizes a proprietary circuit design which includes a
self-correcting architecture. T his sample-and-hold circuit
corrects for internal errors after the hold command has been
given, by compensating for amplifier gain and offset errors, and
charge injection errors. Due to the nature of the design, the
SHA output in the sample mode is not intended to provide an
accurate representation of the input. However, in hold mode,
the internal circuitry is reconfigured to produce an accurately
held version of the input signal. Below is a block diagram of the
AD781.
Tr acking Mode O ffset—T he difference between the input and
output signals when the SHA is in the track mode.
Nonlinear ity--T he deviation from a straight line on a plot of
input vs. (held) output as referenced to a straight line drawn
between endpoints, over an input range of –5 V and +5 V.
Gain Er r or—Deviation from a gain of +1 on the transfer
function of input vs. held output.
P ower Supply Rejection Ratio—A measure of change in the
held output voltage for a specified change in the positive or
negative supply.
1
2
3
4
8
7
6
5
OUT
V
CC
IN
COMMON
NC
S/H
NC
Sam pled D C Uncer tainty—T he internal rms SHA noise that
is sampled onto the hold capacitor.
X1
H old Mode Noise—T he rms noise at the output of the SHA
while in the hold mode, specified over a given bandwidth.
V
AD781
EE
Total O utput Noise—T he total rms noise that is seen at the
output of the SHA while in the hold mode. It is the rms
summation of the sampled dc uncertainty and the hold mode
noise.
Functional Block Diagram
O utput D r ive Cur r ent—T he maximum current the SHA can
source (or sink) while maintaining a change in hold mode offset
of less than 2.5 mV.
REV. A
–5–
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